string replace with jquery assitance

I am sure there is a better way to do this, but if you are trying to always replace the numbers of that file regardless of what they may be you could use a combination of splits/joins like this:

str = "/folder1/folder2/folder3/IMG_143556_TT.jpg" //store image src in string
strAry = str.split('/') //split up the string by folders and file (as last array position) into array.
lastPos = strAry.length-1; //find the index of the last array position (the file name)
fileNameAry = strAry[lastPos].split('_'); //take the file name and split it into an array based on the underscores.
fileNameAry[1] = '987654'; //rename the part of the file name you want to rename.
strAry[lastPos] = fileNameAry.join('_'); //rejoin the file name array back into a string and over write the old file name in the original string array.
newStr = strAry.join('/');  //rejoin the original string array back into a string.

What this will do is make it so that regardless of what directory or original name of the file name is, you can change it based on the string's structure. so as long as the file naming convention stays the same (with underscores) this script will work.

please excuse my vocab, I know it's not very good heh.

Use a regular expression

var str = '/folder1/folder2/folder3/IMG_123456_PP.jpg';

var newstr =  str.replace(/(img_)(\d+)(?=_)/gi,function($0, $1){
                                                  return $1 ? $1 + '987654' : $0;
                                                });

example at http://www.jsfiddle.net/MZXhd/

Perhaps more comprehensible is

var str = '/folder1/folder2/folder3/IMG_123456_PP.jpg';
var replacewith = '987654';
var newstr = str.replace(/(img_)(\d+)(?=_)/gi,'$1'+replacewith);

example at http://www.jsfiddle.net/CXAq6/